Add method to switch to shutdown mode

[jose1711]

From datasheet:

The devices include a 150μA low-power shutdown mode..
..
Shutdown Mode
When the MAX7219 is in shutdown mode, the scan oscillator is
halted, all segment current sources are pulled to ground,
and all digit drivers are pulled to V+, thereby blanking the
display. The MAX7221 is identical, except the drivers are
high-impedance. Data in the digit and control registers
remains unaltered. Shutdown can be used to save power
or as an alarm to flash the display by successively entering
and leaving shutdown mode. For minimum supply current
in shutdown mode, logic inputs should be at ground or V+
(CMOS-logic levels).

It is possible to initiate the shutdown mode by calling a private method:

display._write(const(12), 0)

but calling a public method (e. g. display.shutdown) would be nicer.